The present invention relates to the technology to improve a directivity of a speaker array and a microphone array and, more particularly, the technology to improve a directivity in a low frequency range.
The technology to form a sound field only in a particular direction or pick up a sound arriving only from a particular direction by using the speaker array or the microphone array, which is constructed by aligning a plurality of transducers such as speakers or microphones linearly at a predetermined interval, has spread popularly.
By the way, in the speaker array and the microphone array of this type, it is desired that the same directional characteristic can be realized over a wide band from a high frequency range to a low frequency range. In this case, the directional characteristic in a low frequency range can be improved as an array length (a value obtained by multiplying the number of transducers by an aligned interval of the transducers) of the speaker array or the microphone array is set longer (see Non-Patent Literature 1). Therefore, such a problem existed that, in order to ensure the enough directivity in a low frequency range, a device size of the speaker array and the microphone array is inevitably increased.
Therefore, the technologies to solve the above problem have been proposed variously in the prior art, and the technology disclosed in Non-Patent Literature 2 may be listed as an example. In this Non-Patent Literature 2, the technology to expand the band, which is able to provide the same directional characteristic, toward the low frequency range side by setting filter coefficients of respective digital filters such that the amplitude characteristic of the digital filter connected to each transducer constituting the speaker array or the microphone array becomes equal to the amplitude characteristic (or its approximate characteristic) of the Dolph-Chebychev filter, whose section taken in a two-dimensional frequency plane in the spatial frequency direction gives the stop band equal ripple characteristic, is disclosed.    [Non-Patent Literature 1] Toshiro Ohga, Yoshio Yamazaki and Yutaka Kaneda, “Acoustic System and Digital Signal Process” IEICE 1993-05 pp. 176-186    [Non-Patent Literature 2] Yasushi Matsumoto, Kiyoshi Nishikawa, “Approach of Designing a Directional Array Speaker with a Predetermined Side Lobe Amount” IEICE, Technical Report 2004-74 pp. 13-18
However, normally the ripples having the stop band equal ripple characteristic exist in areas except the non-physical area (area in which |f2|>ρ|f1| is satisfied in a two-dimensional frequency plane. Where ρ=D/cT, T is sampling interval, D is interval of speakers, and c is sound velocity f1 is normalized time frequency, and f2 is normalized spatial frequency.). Therefore, if a large amplitude is given to the stop band equal ripple to improve the directivity in a low frequency range, such a problem arose that an amplitude level of the side lobes that generate the essentially unnecessary directional characteristic is increased.